1. Field of the Invention
This invention relates to a process and apparatus for agglomerating high melting temperature ashes produced in a wide variety of fluidized bed processes including combustion and gasification of coal, other fossil or biomass fuels and waste materials. More specifically, this invention relates to the creation of a plurality of high temperature zones within a fluidized bed in a fluidized bed process to melt ashes produced in the process having a melting temperature above about 2000.degree. F., forming sticky ash particles which adhere to each other to form ash agglomerates. These ash agglomerates can then be withdrawn using density/size selective solids withdrawal.
2. Description of the Prior Art
A significant problem in the operation of high temperature fluidized bed processes such as fluidized bed gasification of coal, fossil or biomass fuels and waste materials is the fusion of ash particles to form large agglomerates in the fluidized bed causing occlusion of the reactor unless they are removed. At least one solution to this problem is disclosed by U.S. Pat. No. 4,315,758 which teaches an inverted conical withdrawal section (hereinafter referred to as a perforated sloping support grid) positioned in the bottom of the fluidized bed reactor having a central opening in communication with a venturi-type nozzle through which is injected a high velocity air/steam stream. In the center of the nozzle is positioned a central jet pipe which extends above the constricted center section and through which an oxygen-containing gas is injected into the fluidized bed. According to the teachings of this patent, the tendency for ash to sinter and occlude in the nozzle and central opening of the perforated sloping support grid is controlled, if not eliminated, by passing the oxygen containing gas into the nozzle through the central jet pipe. U.S. Pat. No. 4,854,249 discloses a two stage combustion process, the first stage of which is a fluidized bed in which carbonaceous materials are combusted producing ash and combustion gases. The fluidized bed is supported on a perforated sloping support grid having a central opening in communication with a constricted central nozzle through which oxygen is injected into the fluidized bed forming a density/size selective solids withdrawal system. U.S. Pat. No. 4,229,289 discloses a fluidized bed process and apparatus having multiple perforated sloping support grids, each having a central opening in communication with a constricted central nozzle through which oxygen is injected forming multiple density/size solids withdrawal systems. Generally, the range of carbonaceous materials which can be processed in a fluidized bed reactor in a non-agglomerating mode is very broad. In the processes disclosed by the '758, '249 and '289 patents, however, the range of carbonaceous materials which can be processed in the agglomerating mode is limited to those materials which produce ash with ash-softening and melting temperatures near the bulk-bed temperature required by the particular carbonaceous material. This is due to the limited differential, on the order of a few hundred degrees Fahrenheit, between the hot zone temperature created in the fluidized bed by the injection of the oxygen-containing gas through the central opening in the bottom of the perforated sloping support grid and the bulk-bed temperature. U.S. Pat. No. 4,693,682 discloses a process for thermal treatment of solid particles within a fluidized bed having a selective heavier particle discharge conduit in communication with a sloping bed support and providing a discrete fluid fueled flame in close proximity to and above the opening to the heavier particle discharge conduit. The flame provides a single higher temperature zone in and around the flame having a temperature between about 100.degree. F. to 400.degree. F. above the temperature of the remainder of the fluidized bed. However, the higher temperature zone created by the flame does not provide the substantial temperature differential between the temperature of this higher temperature zone and bulk-bed temperature required to agglomerate ashes having high melting temperatures substantially above the temperature of the fluidized bed.
Processes which utilize fluidized beds and fluidized bed reactors are well known to those skilled in the art. U.S. Pat. No. 4,955,942 discloses a fluidized bed combustor having a bank of boiler tubes positioned within the bed. The bed material is supported on a flat preformated floor through which air is injected for fluidization of the bed. Fuel for heating the boiler tube bank is injected through burners positioned on the periphery of the combustor into the fluidized bed. U.S. Pat. No. 4,021,184 discloses a fluidized bed waste incinerator in which air is supplied to a wind box positioned below a flat constriction plate on which the bed is supported and having peripherally mounted fuel guns which penetrate the incinerator wall above the constriction plate for furnishing fuel to the incinerator chamber. U.S. Pat. No. 4,308,806 discloses a fluidized bed incinerator having a sloping bottom plate with a central opening and a burner for start-up of the incinerator positioned through the side wall of the incinerator above the sloping plate. U.S. Pat. No. 4,017,253 discloses a fluidized bed calciner in which heat is provided by a combustion nozzle contained within a tube or shroud which extends through the side wall of the calciner into the fluidized bed. Fuel and oxidant are mixed and combusted within the shroud and, due to the shroud, the fluidized bed particles are isolated from the high-velocity, high temperature portions of the resulting flame, thereby reducing particle attrition. U.S. Pat. No. 4,831,944 discloses a process and device for destroying solid waste by pyrolysis in which waste is introduced into the top of a reactor and flows downward counter to the flow of hot gas which is blown in at the base of the reactor through plasma jets positioned on the periphery of the reactor. A boiler having two fluidized beds, an upstream fluidized bed of sand in which fuel is combusted with air fed into the bed as fluidizing gas and a downstream fluidized bed of particulate limestone for desulfurizing the flue gases from the upstream bed, is disclosed by U.S. Pat. No. 4,815,418. A fluidized bed furnace having a distributor plate with fuel chambers and air tubes in communication with said fuel chambers which extend upward into a fluidized bed is disclosed by U.S. Pat. No. 3,914,089. U.S. Pat. No. 4,262,611 discloses a method and apparatus for waste incineration in which waste material is fed into a vessel having an upper pyrolysis chamber and a lower solids incineration chamber separated by a moveable gate. The waste material is subjected to volume reduction in the upper pyrolysis chamber after which it is discharged into the lower solids incineration chamber in which it is combusted. The resulting ash is collected in a frame at the bottom of the lower solids incineration chamber, which frame is removed periodically and the ash contained therein discarded. In the apparatus disclosed by U.S. Pat. No. 3,397,657, waste material is burned in a fluidized bed supported on a first distribution plate positioned above a first windbox and the non-flammable constituents thereof separated and discharged through a central discharge positioned below a second distribution plate, which plate, together with a second windbox above which it is positioned, is centrally positioned above the first distribution plate.
Of the prior art cited hereinabove, only the '758, '289, '682 and '249 patents disclose agglomeration in a fluidized bed system. In the processes disclosed by these patents, the range of carbonaceous materials which can be combusted therein is limited by the small differential, on the order of only a few hundred degrees, between the bulk-bed temperature and the hot zone temperatures within the bed in which the ash is softened and begins to agglomerate. To broaden the range of carbonaceous materials which can be combusted in an agglomerating mode, independent control of the bulk-bed temperature and hot zone temperature is required. None of the prior art of which we are aware discloses or suggests such independent control of bulk-bed temperature and hot zone temperature within a fluidized bed.